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While previous studies have examined the growth and yield response of rice to continued increases in CO2 concentration and potential increases in air temperature, little work has focused on the long‐term response of tropical paddy rice (i.e. the bulk of world rice production) in situ, or genotypic differences among cultivars in response to increasing CO2 and/or temperature. At the International Rice Research Institute, rice (cv IR72) was grown from germination until maturity for 4 field seasons, the 1994 and 1995 wet and the 1995 and 1996 dry seasons at three different CO2 concentrations (ambient, ambient + 200 and ambient + 300 μL L–1 CO2) and two air temperatures (ambient and ambient + 4 °C) using open‐top field chambers placed within a paddy site. Overall, enhanced levels of CO2 alone resulted in significant increases in total biomass at maturity and increased seed yield with the relative degree of enhancement consistent over growing seasons across both temperatures. Enhanced levels of temperature alone resulted in decreases or no change in total biomass and decreased seed yield at maturity across both CO2 levels. In general, simultaneous increases in air temperature as well as CO2 concentration offset the stimulation of biomass and grain yield compared to the effect of CO2 concentration alone. For either the 1995 wet and 1996 dry seasons, additional cultivars (N‐22, NPT1 and NPT2) were grown in conjunction with IR72 at the same CO2 and temperature treatments. Among the cultivars tested, N‐22 showed the greatest relative response of both yield and biomass to increasing CO2, while NPT2 showed no response and IR72 was intermediate. For all cultivars, however, the combination of increasing CO2 concentration and air temperature resulted in reduced grain yield and declining harvest index compared to increased CO2 alone. Data from these experiments indicate that (a) rice growth and yield can respond positively under tropical paddy conditions to elevated CO2, but that simultaneous exposure to elevated temperature may negate the CO2 response to grain yield; and, (b) sufficient intraspecific variation exists among cultivars for future selection of rice cultivars which may, potentially, convert greater amounts of CO2 into harvestable yield.

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Recent anthropogenic emissions of key atmospheric trace gases (e.g. CO2 and CH4) which absorb infra‐red radiation may lead to an increase in mean surface temperatures and potential changes in climate. Although sources of each gas have been evaluated independently, little attention has focused on potential interactions between gases which could influence emission rates. In the current experiment, the effect of enhanced CO2 (300 μL L–1 above ambient) and/or air temperature (4 °C above ambient) on methane generation and emission were determined for the irrigated tropical paddy rice system over 3 consecutive field seasons (1995 wet and dry seasons 1996 dry season). For all three seasons, elevated CO2 concentration resulted in a significant increase in dissolved soil methane relative to the ambient control. Consistent with the observed increases in soil methane, measurements of methane flux per unit surface area during the 1995 wet and 1996 dry seasons also showed a significant increase at elevated carbon dioxide concentration relative to the ambient CO2 condition (+49 and 60% for each season, respectively). Growth of rice at both increasing CO2 concentration and air temperature did not result in additional stimulation of either dissolved or emitted methane compared to growth at elevated CO2 alone. The observed increase in methane emissions were associated with a large, consistent, CO2‐induced stimulation of root growth. Results from this experiment suggest that as atmospheric CO2 concentration increases, methane emissions from tropical paddy rice could increase above current projections.

Although the sensitivity of crop species to water stress during reproductive growth stages is well documented, the causes of sterility are not understood. The role of decreased panicle exsertion as a causal factor of spikelet sterility in water stressed rice (Oryza sativa L.) was investigated. Rice cultivar IR36 was grown on a deep silty loam soil under full sprinkler irrigation for 77 days. A line source sprinkler system was then utilized during panicle exsertion and flowering to impose six continuously decreasing irrigation treatments. After the 15‐day treatment period, full sprinkler irrigation was resumed until maturity. Soil moisture extraction and plant water potential were monitored as soil and plant water stress progressed. Panicle exsertion rate was measured on tagged panicles which were later used to assess percent final panicle exsertion and percent sterility. The six irrigation treatments varied in grain yield from 5.0 t ha−1 to about 1 t ha−1 with corresponding increases in spikelet sterility of 16.6 and 74.2%. Panicle exsertion rate decreased linearly with decreasing mean daily leaf water potential. The degree of final panicle exsertion from the flagleaf sheath and percent spikelet sterility both decreased linearly with panicle exsertion rate. All spikelets left unexserted from the flagleaf sheath were sterile. Panicle exsertion rate was slightly confounded by tiller age in that late tillers in the well irrigated treatment had a slower exsertion rate and higher percent sterility than earlier tillers. The early tillers which exserted at rates of 4.5 to 5.5 cm day−1 under well watered conditions decreased to about 3.0 cm day−1 in the stressed plot and accounted for 25 to 30% spikelet sterility in the severely stressed treatment.

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Increased spikelets sterility is the primary component of yield reduction in rice (Oryzas ativa L.) and other small grains subjected to water stress during reproductive growth stages. The causes of water stress induced spikelet sterility are not clear although the meiotic stage of pollen mother cells is often reported to exhibit yield‐decreasing sensitivity. We exposed rice plants, in which pollen mother cells were actively undergoing meiosis, to the following series of water stress levels as measured by flag leaf water potential: −1.6, −1.9, −2.2, −2.5, −2.8, and −3.5 MPa for 18 to 24 h. Panicles were fixed in Farmer's solution and later examined microscopically for abnormal chromosomal behavior. The occurrence of chromosomal abnormalities such as univalents, lagging of chromosomes, noncongression in metaphase and micronuclei was evaluated in 3822 pollen mother cells in response to increasing water stress levels. Spikelet fertility of comparable panicles grown to maturity decreased linearly from 80% at −1.1 MPa (control) to 50% at −3.5 MPa flag leaf water potential. Chromosomal abnormalities appeared at relatively moderate stress levels (−1.1 to −1.9 MPa) and in general peaked at the −2.2 MPa stress treatment.In general, water stress increased chromosomal abnormalities, except at severe stress levels (−3.5 MPa) where the meiotic process was partially arrested. Each meiotic stage had particular irregularities associated with water stress. Prophase I and II, univalents; Metaphase I, noncongression of bivalents and lagging chromosomes; Anaphase I and II, lagging chromosomes; Telophase II, micronuclei formation. Blasted or immature, small, colorless spikelets also were observed and their occurrence increased with stress level. Blasted spikelets, not included in the calculation of percent sterility, accounted for 10 to 20% of the total spikelets per panicle at moderate stress levels. The cause of this malady is unknown.

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